This invention relates to a hip capitulum cap for a hip joint prosthesis shaped essentially like half a hollow sphere.
Alloplastic hip joint end prostheses are presently used in two thoroughly different embodiments, of which there can be numerous variations and combinations. Familiar first of all are the so-called full prostheses, in which both the natural acetabulum, i.e., the hip socket in the pelvic bones, and the capitulum, or the head of the joint near the neck of the femur of the thigh bone, are replaced with an alloplastic prosthesis, i.e., a hip socket of metal or plastic which replaces the cartilage layer of the acetabulum and is fastened in the pelvic bones by means of bone cement, and a joint head of metal or plastic to which a shaft is integrally attached, the shaft being anchored in the femur medulla cavity by means of bone cement. Such conventional prostheses are disclosed, for example, in West Germman Publication DT-OS No. 1,912,630 and in West German Patent Specification DT-PS No. 1,566,386, FIGS. 1 and 2 thereof.
Since full prostheses are subject to a number of disadvantages, partial prostheses have also become available. In the case of known partial prostheses, only the capitulum, or the capitulum and the acetabulum, are dealt with and fitted with a pan- or bowl-shaped alloplastic, which replaces as necessary, either of the two damaged and artificially removed slide faces, for example, as shown in FIG. 3, of the aforementioned West German Patent Specification DT-PS No. 1,566,386. In this case, there is the possibility of attaching the alloplastic hip capitulum cap either in such a way that it floats loosely on the rounded remaining joint head, e.g., according to Smith-Peterson, or by force-locking it onto the remaining joint head by means of a press fit or form-locking it by means of an anchoring in the joint head and the neck of the femur, e.g., according to Luck. Heretofore, the anchoring was always achieved by means of a shaft adapted to the cap, e.g., in the form of a nail according to Judet or in the form of a screw according to Zanoli. None of the previously tested mountings of the hip head cap have proved entirely satisfactory. Obviously, loosening of the cap must be dealt with in the case of a floating mounting. Loosening of the cap can also occur as a consequence of the underwashing of the cap with synovial fluid, which loosening intensifies the already existing tendency for the formation of a bone tumor (callus) in the neck of the femur due to mechanical irritation by the edge of the cap.
With regard to a force-locked connection, the disadvantages of the floating mounting are only slightly alleviated and not entirely eliminated. If the cap loosens even once, the same conditions arise as in the case of a floating mounting. The disadvantages of a form-locked connection are somewhat similar to those of a full prosthesis, i.e., there is a possibility of damage to the bone tissue due to the polymerization heat emitted upon hardening of the bone cement (e.g. methyl methyacrylate), a considerable amount of which must be used to ensure sufficient anchoring, and of unphysiological or abnormal conditions due to the application of load to the hip joint, which can lead to decomposition of the bone matter, loosening of the prosthesis and bone fractures. In the case of the absence of a hip socket cap, all three methods of fastening share the common disadvantage of gradual destruction of the cartilage layer in the acetabulum due to the relative movement between the acetabulum and the hip head cap.